This invention relates to the controlled humidification of gases used by devices requiring or benefiting from external gas humidification for operation. More particularly, the invention relates to controlling the temperature and humidity of fuel and oxidant gases being provided to a fuel cell under testing conditions.
Humidification of the fuel gas, oxidant gas or both the fuel and oxidant gases is generally required for fuel cells that use solid polymer electrolyte membranes. Proton exchange membrane (PEM) fuel cells require water to support proton conduction through the membrane. While water is a product of fuel cell reactions involving hydrogen or methanol as a fuel and oxygen or air as an oxidant, the amount of water formed is often inadequate to maintain membrane hydration.
One reason for the lack of sufficient hydration of proton exchange membranes in fuel cells is that the water is formed at the electrode where the oxidant is consumed and water is carried away by electroosmosis from the fuel consuming electrode. A significant amount of the water produced in the fuel cell reaction is removed from the fuel cell (either as water vapor or liquid water) by the flowing, heated oxidant gas stream, typically air. During operation of a PEM fuel cell, water is continually transported across the proton exchange membrane from the fuel consuming electrode to the oxidant consuming electrode due to electroosmosis.
While the product water formed may maintain sufficient humidification of proton exchange membranes at low and intermediate current density conditions, the membrane can dry out and experience increases in its internal resistance at high current density conditions. The problem of the membrane drying out has typically been addressed by adding water, usually as water vapor, to the gas stream containing the fuel, or to both gas streams (fuel and oxidizer). It also should be noted that the performance of the fuel cell decreases if the catalyst layer is flooded with liquid water either from excess water vapor being delivered to the fuel cell or the lack of a means of removing sufficient product water.
Various methods of introducing water directly in the fuel cell have been developed. U.S. Pat. No. 5,262,250 (Watanabe) teaches the use of narrow paths or wicks within the proton exchange membrane for maintaining hydration of the membrane in a fuel cell stack. However, a PEM with wicks is difficult to manufacture, requires increased manifold requirements for the cell frames, requires generation and delivery of water to the paths, presents difficulties in delivering the water uniformly across the surface of the membrane, and the amount of flow of liquid water that can be achieved through the membrane is limited and uncontrolled. In addition, the wicks rely on wetting to promote fluid flow.
Another method that is commonly used is to humidify a reactant gas inside the cell assembly, or stack, itself. This is usually done with a membrane humidifier. In this type of humidifier, a stream of liquid water is located on one side of a water permeable membrane while the reactant gas stream flows on the other side. This method uses the heat of the fuel cell itself to evaporate the water. This eliminates the need for separate heaters to humidify the reactant gas streams, but it limits the humidification of the gas streams to a dew point that is essentially the same as the cell""s operating temperature. It also adds to the size of the cell stack. Since the humidifier is a structural part of the stack, it has to be built to serve as a supporting member. This can increase the weight and size of the system by a greater amount than is required for an external humidification system.
Another method for humidifying a PEM is to inject liquid water directly into either the manifold of the cell (or stack), or a reactant gas line leading to the manifold. The liquid water is injected in such a manner as to form a mist in the reactant gas line. As the reactant gas stream is heated by the cell, the water quickly evaporates due to the high surface area resulting from small droplet sizes. This type of humidifier produces a very compact humidification system. The amount of water vapor in the reactant gas stream can easily be controlled by metering the liquid water into the cell. While this can be a good system for stacks in the kilowatt range and larger, it is not an effective system for smaller fuel cell systems. The weak point of water injection methods is the difficulty encountered in forming a steady and consistent mist at low liquid water flow rates. For instance, a nominally 1 kW stack consisting of six cells, each at 0.6 V, operating at 85xc2x0 C. with both the fuel and air streams humidified, requires about 10.3 grams of water per minute to humidify its air stream, assuming a 2:1 air to current stoichiometry (meaning two times the theoretical amount of air needed) at 30 psig. This amount is easily metered on a consistent basis. A smaller stack, generating 300 W at 70xc2x0 C. requires only 1.50 grams of water per minute under the same feed conditions. This flow rate of water can be metered, but the higher precision required to maintain a smooth flow at the lower feed rate results in the smaller stack actually requiring a more complex humidifier. In the case of a small single cell operating at 30 W, and the same operating conditions as above, the feed rate drops to 0.150 grams of water per minute for the air stream and even less for the fuel gas stream. At these rates, maintaining a steady flow rate of water is extremely difficult.
The simplest way to humidify a reactant gas stream is to pass the gas as a stream of fine bubbles through a column of liquid water. As long as the gas has sufficient contact time with the water, the amount of water vapor in the reactant gas stream can be controlled by controlling the temperature of the liquid water. This method works well at low gas flows. To fully saturate the reactant gas with water vapor requires either small bubbles, ideally under 0.5 mm in diameter, or a tall column to allow enough contact time to ensure complete saturation. Operating the humidifier under conditions in which the reactant gas does not have sufficient contact time to become fully saturated and, as a result, is carrying a varying amount of water vapor leads to unrepeatable operation, reduced performance, and possibly damage to the fuel cell. For example, if a contact time of 0.5 seconds is required to saturate the reactant gas bubbles with water, the column will need to be at least 19 cm tall (based on Stokes law velocity of 38.2 cm/sec for a 0.5 mm bubble of air in water at 80xc2x0 C.). For a flow rate of one liter of reactant gas per minute forming 0.5 mm bubbles with an average spacing of 0.5 mm, a liquid water volume of over 300 cm3 is required, with a similar or greater volume for the reverse portion of the convective flow produced by the reactant gas lifting the liquid water. Additional volume is required for the disperser to form the bubbles and for a reserve of liquid water to replenish that lost to evaporation. The resulting humidifier has a volume of over one liter, and any increase in reactant gas flow will require an even larger volume.
U.S. Pat. No. 5,512,831 to Cisar teaches an internal humidification device that uses an external humidifier system and a water permeable membrane. A set of parallel water permeable tubes are used to controllably humidify a reactant gas fed to a fuel cell. The humidity is controlled by controlling the temperature at which the humidification occurs and/or by controlling the reactant gas flow rate through the system. The humidification capacity of the system is limited by the amount of liquid water that can pass through the walls of the water permeable membrane tubes. The water transfer rate is varied by adjusting the water temperature and the gas flow rate. However, the reactant gas flow rate is generally set at the rate required to operate the fuel cell under specified conditions, leaving the liquid water temperature as the only variable means of increasing or decreasing the humidification level. Due to the thermal mass of the system and the volume of water in contact with the membrane, rapid increases of the liquid water temperature are difficult to achieve, and decreasing the water temperature requires a water cooling system. The total quantity of water entrained in the reactant feed gas and the rate of water transfer are unknown and must be approximated based on other operating conditions.
U.S. Pat. No. 5,368,786 (Dinauer) teaches a humidification method using porous stainless steel tubes. The porous stainless steel tube system experiences the same limitations inherent to the membrane tube system, and may allow free reactant gas passage through the porous steel tubes if a system upset occurs and the liquid water level or water pressure within the tubes is not maintained at its proper value.
Thus, there remains a need for an improved gas humidification system and method for use with fuel cells. It would be desirable if the humidification system could form part of a stable system for testing, evaluating and utilizing fuel cells over a wide range of operating conditions and reactant gas flow rates that provides real-time, accurate control of the quantity of water delivered to the fuel cell.
The present invention provides an apparatus for controlling the temperature and humidity of a gas stream. The apparatus comprises: an evaporator chamber having a water inlet flow control valve, a heater and a port for the delivery of water vapor and the return of condensate; a gas humidifying chamber in fluid communication with the evaporator chamber port, the gas humidifying vessel having a gas inlet, elements arranged in the vessel to return condensate to the evaporator chamber port and a humidified gas outlet; and a gas transfer line in fluid communication with the humidified gas outlet, the gas transfer line having an outlet and a heater for providing the gas to the outlet at a setpoint temperature. It is preferred that the evaporator chamber port provides restricted fluid communication between the evaporator chamber and the gas humidifying chamber, such as through an orifice, in order to substantially prevent passage of the gas into the evaporator chamber during use. The apparatus may include a temperature sensor in thermal communication with the humidified gas outlet from the gas humidifying vessel and/or in thermal communication with the outlet from the gas transfer line. The elements are selected from baffles, trays, packing and combinations thereof and should be of a suitable design to direct condensate from the gas humidifying chamber back to the evaporator chamber.
The invention also provides a method for humidifying a reactant gas stream for use in a fuel cell. The method comprises: passing the reactant gas stream through a humidification chamber at a flow rate consumed by the fuel cell; providing liquid water into an evaporation chamber to produce water vapor at a flow rate proportional to the reactant gas stream flow rate; communicating the water vapor from the evaporator chamber through an orifice into the humidification chamber for humidifying the reactant gas stream; and maintaining the temperature of the humidified gas stream above a set point temperature.